rust-locator-patterns

PET Rust Locator Patterns & Conventions

Domain knowledge for writing correct Rust code in the Python Environment Tools (PET) repository. Following these patterns prevents the most common review feedback and CI failures.

Locator Ordering (CRITICAL)

The order of locators in create_locators() in crates/pet/src/locators.rs determines identification priority. More specific locators MUST come before generic ones.

Priority Chain (required order):

1. Windows-specific: Windows Store → Windows Registry → WinPython
2. Managed environments: PyEnv → Pixi → Conda
3. Virtual envs (specific → generic): Poetry → PipEnv → VirtualEnvWrapper → Venv → VirtualEnv
4. macOS-specific: Homebrew → MacXCode → MacCmdLineTools → MacPythonOrg
5. Linux fallback: LinuxGlobalPython (MUST BE LAST)

Why it matters: If Venv runs before Poetry, a Poetry environment (which IS a venv) gets misidentified as a plain venv. The first locator whose try_from() returns Some wins.

Environment Identification (try_from)

Each try_from() must be precise enough to avoid false positives:

• Poetry: Match {project-name}-{8-char-hash}-py{major}.{minor} naming pattern AND verify location is in poetry cache dir
• Pipenv: Look for .project file in centralized dir with hash-based naming
• Conda: Require conda-meta/ directory with history file
• Pixi: Require .pixi/envs/ in path structure
• venv: Check for pyvenv.cfg file (but many env types have this — venv is a fallback)
• VirtualEnv: Activation scripts WITHOUT pyvenv.cfg (most generic, last resort)

Red flag: A try_from() that only checks pyvenv.cfg existence — that matches ALL venvs, not just one type.

JSONRPC Protocol Rules

The server communicates via stdin/stdout. Any stdout pollution breaks the protocol.

• NEVER use println! — it writes to stdout and corrupts JSONRPC messages
• Use log crate macros: info!(), trace!(), warn!(), error!()
• Log with elapsed time for diagnostic handlers (established pattern)
• All tracing/logging writes to stderr via subscriber configuration

Thread Safety Patterns

PET uses heavy concurrency for parallel discovery:

• Shared state requires Arc<Mutex<T>> or Arc<RwLock<T>>
• Keep lock scopes minimal — clone data out, drop lock, then process
• Use .expect("context") on mutex locks, not bare .unwrap()
• No nested locks (deadlock risk)
• Consider thread::scope for structured concurrency

Platform-Specific Code

Use #[cfg(...)] attributes for conditional compilation:

#[cfg(windows)]
use pet_windows_registry::WindowsRegistry;

#[cfg(unix)]
fn resolve_path(p: &Path) -> PathBuf { std::fs::canonicalize(p).unwrap_or(p.to_path_buf()) }

#[cfg(target_os = "macos")]
use pet_homebrew::Homebrew;

Do NOT use if cfg!(windows) { ... } for code that imports platform-specific modules — the code inside still gets compiled on all platforms.

Platform Gotchas:

• Windows: Avoid canonicalize() for directory junctions (breaks Scoop symlinks). Use norm_case() instead.
• macOS: Homebrew has complex symlink chains; resolve_symlink before canonicalize
• Linux: /bin may symlink to /usr/bin — handle both

Version Detection (Prefer File-Based)

Order of preference (cheapest first):

1. pyvenv.cfg — version or version_info field
2. conda-meta/python-*.json — package metadata
3. Include/patchlevel.h — CPython source header
4. Parse from executable path (e.g., python3.11)
5. Spawn Python --version — LAST RESORT (expensive, defeats PET's purpose)

Path & Symlink Handling

• Use pet_fs::path::resolve_symlink() — the project's own resolver
• For comparison: canonicalize().unwrap_or(original) then norm_case() for Windows UNC prefix handling
• Preserve original user-facing paths; resolve only for identification

Testing Conventions

• Feature-gated CI tests: #[cfg_attr(feature = "ci", test)] with #[allow(dead_code)]
• Feature flags: ci, ci-poetry-global, ci-jupyter-container, ci-homebrew-container
• Verification pattern: spawn Python, use try_from(), test symlinks, use resolve
• Use cargo test --features ci for CI-gated tests

Serialization

• All JSON-serialized structs use #[serde(rename_all = "camelCase")]
• Matches the JSONRPC API conventions consumed by VS Code Python extension

Pre-Commit Requirements

Always run before committing:

cargo fmt --all
cargo clippy --all -- -D warnings

Never suppress clippy warnings with #[allow(...)] without justification.

Conda-Specific Patterns

• Conda manager detection takes precedence over mamba: get_conda_manager(path).or_else(|| get_mamba_manager(path))
• Managers reported separately (Conda, Mamba) but environments remain PythonEnvironmentKind::Conda
• Support detection from history files and conda-meta directories